The fluorocarbons chlorotrifluoromethane and dichlorodifluoromethane have received extensive use in refrigerants, foam blowing agents and other applications. They are members of a group of chlorofluorocarbons referred to as FC or CFC 11 and 12, respectively. Because of their stability and long atmospheric lifetime, there has been concern about possible effects of these fully halogenated fluorocarbons on stratospheric ozone. This concern has spurred the search for replacements that are free of chlorine or, due to the presence of a reactive hydrogen, will have a much shorter atmospheric lifetime. A notable candidate, particularly for the replacement of FC 12, is 1,1,1,2,-tetrafluoroethane, also known as FC 134a.
Unfortunately, synthesis of this compound is difficult, and isomers and a variety of byproducts generally dominate reaction product mixtures. The following is a review of the pertinent known reactions.
The use of trichloroethylene as a starting material is described in two patents to Ruh et al. (Dow Chemical), Nos. 2,745,886 (May 15, 1956) and 2,885,427 (May 5, 1959). The catalyst is an oxygenated chromium (III) fluoride obtained by burning CrF.sub.3 .multidot.3H.sub.2 O in oxygen at 600.degree. C. The highest yield reported for 1,1,1,2-tetrafluorethane, however, was only 3% (mole basis).
The product has also been prepared by the fluorination of 1,1,1-trifluoro-2-chloroethane in various processes. One is described by Bell (ICI), U.S. Pat. No. 4,129,603 (Dec. 12, 1978), in which chromium oxide or basic chromium fluoride is used as a catalyst. A metal permanganate is then used to destroy 1,1-difluoro-2-chloroethylene produced as an impurity. A similar disclosure appears in Potter (ICI), U.S. Pat. No. 4,158,675 (June 19, 1979), except that additional hydrogen fluoride at a controlled temperature is used for conversion in place of the metal permanganate. Daikin Kogyo KK in British Patent No. 2,030,981 (Mar. 2, 1983) describe the use of a chromium (III) compound as a catalyst with oxygen fed to the reaction system as a means of preventing loss of catalytic activity. Gumprecht (DuPont) in U.S. Pat. No. 4,311,863 (Jan. 19, 1982) disclose the use of a potassium, cesium or rubidium fluoride catalyst in aqueous solution.
The product has also been prepared from dichlorotetrafluoroethanes by hydrogenolysis, as in the process described by ICI in British Patent No. 1,578,933 (Nov. 12, 1980), where a catalyst consisting of palladium on either an inactivated carbon or an alumina support is used in the presence of hydrogen.
Chromium catalysts are well known for fluorination reactions in general. Scherer et al. (Hoechst) in U.S. Pat. Nos. 3,660,307 (May 2, 1972) and 3,859,424 (Jan. 7, 1975) disclose catalysts prepared by reacting a hydrated oxide of trivalent chromium with hydrogen fluoride to form a trivalent chromoxy fluoride. The catalyst disclosed in Knaak (DuPont), U.S. Pat. No. 3,978,145 (Aug. 31, 1976) is a hexagonal-CrOOH (again including trivalent chromium), while that disclosed in Knaak (DuPont), U.S. Pat. No. 3,992,325 (Nov. 16, 1976) is a gamma-CrOOH plus a metal fluoride. A black chromium oxide with empirical formula Cr.sub.2 O.sub.3-6 obtained by calcining the hydrated oxide in the presence of oxygen is disclosed in Chapman et al. (ICI), U.S. Pat. No. 3,426,009 (Feb. 4, 1969) and Firth et al. (ICI), U.S. Pat. No. 3,755,477 (Aug. 28, 1973). This catalyst was only able to form 1,1,1-trifluoro-2-chloroethane, however.
Chromium in combination with alumina has also been disclosed. One example is Japanese Patent No. 40,695 (Central Glass, Sept. 22, 1981) where trivalent chromium on alumina was used for the fluorination of CCl.sub.2 F--CClF.sub.2 to CClF.sub.2 --CClF.sub.2. Ruh et al. (Dow), U.S. Pat. No. 2,744,148 (May 1, 1956) disclose a catalyst consisting of aluminum fluoride, alumina and a metal halide in which the metal may be cobalt, chromium or others. Japanese Patent No. 24,925 (Central Glass, July 24, 1978) discloses a catalyst prepared by impregnating gamma-alumina with TiCl.sub.3 which is then fluorinated. Milks (Dow), U.S. Pat. No. 2,744,147 (May 1, 1956) discloses a catalyst consisting of cobalt, nickel or chromium fluoride on an alumina support.